It's because you love him
Dean escapes from Bobby's strong room with every intention to find Michael and say yes for his use as a vessel. Castiel tracks him down and beats him up in anger and then brings him back to Bobby's.
"Oh my God, what happened to him?" cried Sam, shocked. Castiel had appeared out of thin air, supporting his unconscious brother.
"Me." growled Castiel as he unceremoniously deposited Dean's body on Bobby's futon. "Where's Adam?"
Sam shot a quick look at Bobby, "Yeah about that…" The younger Winchester quickly explained how the angels had made off with his half-brother in a dream.
Castiel looked annoyed. In fact, Sam thought he heard the angel mutter a swear under his breath.
"What do we do about Dean? Is he going to be alright?" said Sam, his brow furrowed with concern.
"He's fine." spat Castiel. "You can cuff him to the pipe in Bobby's stronghold."
…...
A few hours later Sam made his way down the stairs with a platter. He opened the door of the strong room carefully. Dean was still unconscious, handcuffed to the pipes as Castiel had ordered. Castiel himself was half-perched, leaning against boxes in a corner of the room. The trench coat clad figure was so hidden in the dark that Sam didn't even notice him at first.
"Oh, Castiel. What are you doing here?"
"I don't have anywhere else to go." he said quietly.
Sam was suddenly struck by how lonely he sounded. He put the platter down. "So Dean's not up yet?" he asked tentatively.
"No." said Castiel, "He's not going to wake up for some time now."
"Okay. What exactly did you do to him?" said Sam slowly.
The angel sighed and looked away, "I was angry." he muttered. "But I shouldn't have hit him. That was wrong of me. It was just so…I felt so…" he stopped abruptly, his lips twisted in a grimace, a look of anger and reproach on his face as if he had just stopped himself in time from saying something he shouldn't have said. "Why does he have to be so difficult, your brother?" he ended, sighing exasperatedly.
Sam let out a short laugh, "Oh, well. That's Dean for you. He's never been the easiest of people to be around with."
"I don't even know what I'm doing any more." said Castiel, sounding morose.
"Hey, Castiel." Sam said softly, disconcerted by the change in the nerd angel. "I know this sucks. I mean rebelling against your folks, that too for a seemingly lost cause. Hell, I of all people know the feeling. Half the time I didn't know what I was doing, or why I was doing it."
"Why did you do it, Sam?" said Castiel, his blue eyes piercing his.
"I…don't know. It just felt like the right thing to do at the time. I'm not proud of it." said Sam quickly.
The angel nodded slowly, still looking confused. He turned around, pacing, his back to Sam's.
"Castiel?" said Sam suddenly.
"What?"
"Why did you?" Sam breathed.
"Why did I what?" said Castiel irritably.
"Why did you rebel?" breathed Sam.
Castiel stopped in his tracks. "I don't know."
"Because Dean asked you to?" Sam pressed on.
Castiel snorted. Dean made a slight movement, a whimper escaping from his breath, as his body tried to shift around unsuccessfully. He was still dead to the world. Castiel shot him a look of deep disgust.
'It's because you love him, isn't it' Sam thought as he saw the angel pace back towards his brother's unconscious form and bend to place two fingers to the side of his neck.
The realisation hit the younger Winchester like a comet exploding in his face. Somewhere shadowed by the sudden displays of loathing and violence and intense eye-contact and sheer frustration towards Dean Winchester, lay hidden the angel's true feelings. This seemingly asexual, emotion-less ball of 'I don't understand that reference', wrapped in a beige trench coat of sarcasm failure was in love with his brainless (and currently unconscious) git of a brother. Castiel had rebelled for Dean Winchester. He had fallen for Dean Winchester. And the more he thought about this, the more it became obvious to Sam. This broken angel was hovering in Bobby's strong hold with nowhere to go, because of Dean Winchester. And Dean Winchester did not know.
Sam sighed. The angel was busy muttering mysterious chants in Enochian into his brother's ears. He straightened up abruptly, "He should awake in a few minutes." He turned around.
"Hey hey hey, where are you off to?" Sam cried.
"I have to be somewhere." Castiel said shortly before disappearing.
"Don't li…" Sam opened his mouth, but the angel was already gone, "Damn you Castiel!"
Dean stirred beside him, mumbling something. His speech was slurred and his head felt woozy. He opened his eyes and blinked a couple of times, his eyes adjusting to the darkness in the room.
"Hiya Sammy, " he said hoarsely. He tried to get up and noticed that he was chained to the pipe. "Brilliant." he muttered.
Dean's eyes swept the room again, "Where's Cas?" he asked, his green eyes narrowed in a frown. After everything, he had expected to wake up and find the angel sitting across from him, his nose inches from Dean's, glowering in his customary manner.
"Uh, he's gone. Said he had to be somewhere." said Sam awkwardly.
"Damn." Dean mumbled, shuffling his hand to loosen the grip of the handcuff, "That son of a bitch." he sighed, catching Sam's eyes, "I let him down." He blinked a couple of times, turning his head away.
"Yeah. Is that why he beat the crap out of you?" said Sam. He felt like rolling his eyes in derision.
"Oh yeah! Boy was he mad." said Dean half-laughing. He stopped. His ribs were hurting.
"Yeah, well. There's food over there if you want." said Sam getting up.
"Wait wait wait, aren't you going to un-cuff me?" Dean yelled.
"No."
"Why the hell not?"
Sam walked towards the door and opened it, pausing to look back towards his brother, "Dean?"
"Yeah?"
"Next time don't piss off the nerd angel." Sam shut the door behind him.
…...
The younger Winchester made his way up the stairs. Bobby greeted him at the landing with a bottle of beer. Sam grabbed it and began gulping it down.
"So the two of them kissed and made up yet?" said Bobby.
Sam choked on his beer. Resting one hand on the railing he tried to catch his breath. "Wh..what?" he wheezed, beer dribbling down his nose and chin.
"Oh don't pretend you haven't noticed." Bobby snapped, thumping his hand on Sam's back.
"What do we do, Bobby?" said Sam earnestly, his pale green eyes, full of hope, boring into Bobby's.
"This is Dean, we're talking about. He's an idjit! We do nothing. We hope to Gawd we can end the apocalypse and that we all survive. That'll do for now." Bobby wheeled himself away.
Sam finished the rest of his beer with reluctance and went to the kitchen to throw the bottle. He stopped at the sink to wash his face and shirt. 'They'll have peace when you are done, Sammy.'
That's enough to go on, for now.